This invention relates generally to television transmission and reception systems and, more particularly, to television signal encoding and decoding systems providing secure transmission and reception of both video and audio components of a television signal.
Secure transmission of television signals has become a matter of increasing importance with the growing popularity of video teleconferencing, cable TV and satellite TV transmissions, and with the advent of direct-broadcast-satellite (DBS) transmission. Various television signal encoding techniques have been developed that provide varying levels of security, with corresponding levels of complexity and cost. These range from relatively simple but easy to "break" sync-suppression techniques, which can be employed to protect the video component of a television signal, to relatively complex but difficult to "break" digital encryption techniques, which can be employed to protect both the video and audio components of a television signal. One video encoding technique that provides a relatively secure video signal, with a modest amount of complexity and cost, is line spin scrambling.
Line spin scrambling is performed in an encoder by segmenting the active portion of each video line at a breakpoint determined by a pseudorandom number generator. The two segments of each video line are then interchanged, or "rotated," while the horizontal and vertical synchronization and blanking intervals are left intact. After transmission and reception of the video signal, the signal is unscrambled in a decoder by reversing the line spin scrambling applied to each video line in the encoder. The spin breakpoint of each scrambled video line is determined in the decoder by an identical pseudorandom number generator that is synchronized with the pseudorandom number generator in the encoder. A typical line spin scrambling system is disclosed in U.S. Pat. No. 4,070,693 to Shutterly.
Although line spin scrambling offers many advantages, it has certain disadvantages. One of the disadvantages is that an amplitude gap or discontinuity appears at the point in each scrambled video line where the two segments are pieced together. The amplitude gap results from the difference in the amplitudes at the beginning and the end of the active portion of each video line prior to line spin scrambling. This is because, after scrambling, these two amplitudes are positioned at the same point in the video line. The amplitude gap provides the location of the spin breakpoint in each scrambled video line and, therefore, allows a pirate to unscramble the scrambled video signal.
In addition to providing a means for unscrambling the scrambled video signal, the amplitude gap or discontinuity in each scrambled video line also causes a distortion in the unscrambled video line. The distortion results from the very high frequency content of the discontinuity and the limited bandwidth of conventional television transmission and reception systems The distortion occurs during the transmission and reception process and appears in the vicinity of the discontinuity in each scrambled video line. As a result of unscrambling, the distortion is shifted to the beginning and the end of the active portion of the unscrambled video line.
Another disadvantage of line spin scrambling is a distortion in the unscrambled video signal caused by line tilt. Line tilt is a sawtooth-shaped voltage error that also corrupts each video line during the transmission and reception process. The phase of the sawtooth waveform is such that a linear charge ramp occurs during the horizontal blanking interval and a linear discharge ramp occurs during the active portion of the video line. The linear discharge ramp corrupts the active portion of each video line whether the video line is scrambled with the line spin technique or not. However, the effect on a received picture is generally undetectable when a video signal has not been line-spin scrambled. This is because the amplitude and phase of the line tilt are approximately the same for all video lines and, therefore, the effect across the received picture is constant in the vertical direction and is a gradual luminance variation in the horizontal direction. However, a video line that has been line-spin scrambled has the full amplitude of the line tilt applied at a single point, where the two segments are pieced back together during unscrambling. This causes a sharp luminance discontinuity at the randomly chosen spin breakpoint in each video line, resulting in a chaotic hashing of luminance striations in the received picture.
Although the video component of a television signal can be made relatively secure by line spin scrambling, the audio component of the television signal is preferably made secure by digital encryption techniques. Because of the increasing popularity of stereo television broadcasts, the audio signal is frequently a stereo audio signal. However, a conventional NTSC (National Television System Committee) television signal provides only a single channel for the audio signal on a subcarrier frequency at 4.5 MHz above the video carrier frequency. The 4.5 MHz audio channel is not sufficiently separated from the video upper bandwidth at 4.2 MHz to easily accommodate the bandwidth required for a stereo audio signal.
Various techniques, which vary depending upon the mode of transmission of the television signal, have been devised for transmitting stereo audio within these bandwidth limitations. For ground-based television transmission systems, such as over-the-air or cable transmission systems, the bandwidth of the television signal is generally limited to 4.5 MHz. One technique for transmitting stereo audio within this 4.5 MHz bandwidth limitation is to transmit the stereo audio signal in a burst during the video horizontal blanking interval. However, a digitally-encrypted stereo audio signal cannot be transmitted during the brief horizontal blanking interval unless the digitized signal is either compressed or the sampling rate of the stereo audio signal is reduced below the Nyquist sampling rate. Both of these alternatives severely degrade the quality of the stereo sound. In addition, transmitting an audio signal during the video horizontal blanking interval destroys both the video horizontal synchronization signal and the video color burst signal.
For satellite-based transmission systems, analog stereo audio signals are frequently transmitted over two separate channels at subcarrier frequencies of 5.8 and 7.6 MHz above the video carrier frequency. The two channels are necessary because the conventional NTSC audio channel at 4.5 MHz interferes with the video signal and, therefore, is generally not transmitted. However, the two satellite channels are typically analog channels and cannot accommodate the bandwidth required for transmission of a digitally-encrypted stereo audio signal, unless the digitized signal is either compressed or the sampling rate of the stereo audio signal is reduced below the Nyquist sampling rate.
Accordingly, there has been a long existing need in the broadcasting industry for an improved television transmission and reception system employing line spin scrambling for the video signal and digital encryption for the audio signal. The line-spin scrambled video signal should be capable of being transmitted and received without suffering from the disadvantageous effects caused by amplitude gaps and line tilt. The digitally-encrypted audio signal should be capable of being transmitted and received within the bandwidth limitations of the conventional NTSC or satellite television signal without the digitized audio signal being compressed or the sampling rate of the audio signal being reduced below the Nyquist sampling rate. The present invention clearly fulfills these needs.